FINAL Report

Digital Light-in-Flight Recording by Holography by Use of a Femtosecond Pulsed Laser 2011-12

CHAPTER 1

INTRODUCTION

IN RECENT years, optical communication systems and optical information

processing systems have been more and more required for achievement of high-speed and

high-capacity performances. These optical systems attract much attention because they can

overcome the problems associated with high-speed electronic information and

communication systems such as electromagnetic inference noise, signal delay, clock skew,

and wire congestion. Especially, integrated optical devices are beneficial for optical

interconnects, photonic networks, optical waveguides and so on. In other words, works on

integrated optical devices have been actively researched. Observation and visualization of the

behavior of light pulses in these devices is useful to characterize and evaluate them and to

improve their performance. One of the ways for improvement of performance is to use a

femtosecond pulsed laser as an optical source.

A femtosecond pulsed laser has ultrahigh temporal resolution and is actively used not

only in optical communication but also in material processing, induction of ultra-fast

phenomena, and so on. Therefore, techniques capable of visualizing a femtosecond light

pulse are quite important. Examples of techniques for visualizing light propagation are

femtosecond time-resolved optical polarigraphy (FTOP), photon scanning tunneling

microscopy (PSTM), a streak camera, and light-in-flight recording by holography (LIF

holography). FTOP uses a nonlinear optical phenomenon and indirectly observes

femtosecond light pulses. Because FTOP requires repetitive femtosecond light pulses, it

cannot always correctly observe pulse propagation in the case when repetitive characteristics

of femtosecond light pulses are not uniform. PSTM can visualize femtosecond light pulses by

utilizing evanescent waves by near-field optical microscopy. Also PSTM indirectly visualizes

femtosecond light pulses because it requires numerical estimation by computer.

In addition, visualization by using a single pulse is impossible in principle for FTOP

and PSTM since these techniques require repetitive light pulses or sequential recording.

Dept. Of ECE, KIT, Tiptur 1


Indeed a streak camera can directly visualize femtosecond light pulses, but it can only obtain

1-D information of pulses. On the other hand, LIF holography can record a moving picture of

propagation of femtosecond light pulses by using a single pulse and can obtain 2- or 3-D

information of pulses. Additionally, this technique is capable of directly observing

femtosecond light pulse propagation as a form of a spatially and temporally continuous

moving picture.

These characteristics are also advantages of LIF holography because other techniques

for observing propagation of a femtosecond light pulse require sequential recording with a

delay line or only obtain a moving picture of a few frames. In the past, observation of

picosecond or femtosecond light pulse propagation in air and inside optical devices has been

reported by means of LIF holography. In LIF holography, high-resolution holographic plates

such as Agfa Holotest 8E75HD, which was barely sensitive to near-infrared light, were

mainly used as a holographic recording material. However, production of holographic plate

was discontinued. Although photoconductor plastic hologram (PPH) is one alternative to a

holographic recording material, photosensitivity of the PPH depends on wave-length of light

and requires high energy of light for recording

of a hologram. On the other hand, digital LIF (DLIF) holography which used an image

sensor to record holograms was reported. DLIF holography applies LIF holography to digital

holography that records holograms by an image sensor such as a charge-coupled device

(CCD) or a CMOS image sensor.



CHAPTER 2

LIGHT-IN-FLIGHT RECORDING BY HOLOGRAPHY

2.1 RECORDING PROCESS:

Following figure 2.1 shows a basic recording arrangement of LIF holography. A light

pulse which is generated from an ultra-short pulsed laser is divided into two pulses by a beam

splitter. Two pulses are collimated by microscope objectives and collimator lenses. One

ultra-short light pulse is called as an illuminating light pulse and is introduced into a diffuser

plate with inclined angle.

Fig 2.1 Top view of a basic arrangement of LIF holography.

BS-beam splitter; M-mirror; OB- microscope; objective; CL-collimator lens



Light pulse diffused or scattered by diffuser plate is called as an object light pulse.

Other ultra-short light pulse is called as a reference light pulse and is introduced into a

holographic recording material with an inclined angle. Only when both illuminating light

pulse and reference light pulse arrive at recording material at the same time, interference

fringes are formed by both pulses and recorded on recording material. Reference light pulse

sweeps over recording material in recording process because pulse is introduced into plate

with an inclined angle. Therefore, behavior of the light pulse propagating on diffuser plate at

each instant is recorded in different parts of recording material.

2.2 RECONSTRUCTION PROCESS:

Fig.2.2(a) shows hologram obtained after chemical development of recording

material is set at position where recording material was prepared in recording process. A

continuous wave (CW) laser is used for reconstruction of the moving picture of light

propagation. Wavelength of CW laser is same as that of the ultra-short pulsed laser which

was used in recording process.

Light emitted from CW laser is collimated and illuminates hologram at the same

angle as the reference light pulse. Each different part of hologram reconstructs image of light

propagation at a different instance as shown in fig 2.2(b). Therefore, by moving gazed point

on the hologram along same direction in which reference light pulse swept, one can directly

observe an optical image of a spatially and temporally continuous moving picture of ultra-

short light pulse propagation without any camera.



Fig 2.2(a) Reconstruction.

BS-beam splitter; M-mirror; OB- microscope; objective; CL-collimator lens

Fig 2.2(b) Reconstruction of holograms for obtaining moving picture in DLIF holography.



CHAPTER 3

Digital Light-in-Flight Recording by Holography

In DLIF holography, a hologram is recorded by an image sensor. A reconstructed

image is obtained from recorded hologram by computer. Therefore, instead of moving gazed-

point, images reconstructed from a set of pieces of digital holograms that were extracted

from digitally recorded hologram as shown in Fig. 2.2(a). In this case, relation between

temporal and spatial resolutions is trade-off. Here moving picture can be obtained by

sequentially reconstructing images from holograms.

Definition of Parameters

θ0 -Incident angle of illuminating light pulse against normal of diffuser plate

θR -Incident angle of reference light pulse against recording material

LA-R -Distance between A and R

LO-A0 - Distance between O and A0

LO-R - Distance between O and R

LR-R0 - Distance between R and R0

tA-R -Time required for the object light pulse to propagate from A to R

to-A0 - Time required for the illuminating light pulse to propagate from O to A0

to-R -Time required for the object light pulse to propagate from R to R0

tR-R0 - Time required for the reference light pulse to propagate from R to R0

T0 -Time when the illuminating light pulse arrives at O

TA0 - Time when the illuminating light pulse arrives at A0

TR0-Time when the object light pulse scattered at O arrives at R0

TR - Time when the object light pulse scattered at A arrives at R

τR0 -Time when the reference light pulse arrives at R0



CHAPTER 4

NUMERICAL SIMULATION

DLIF holography which numerically simulated that uses a femtosecond pulsed laser.

Simulation model was based on optical setup shown in Fig. 4(a). The plane on which diffuser

plate was set and axis that was perpendicular to the diffuser plate as xy-plane and z-axis,

respectively. Behavior of light was calculated by means of ray tracing. Coordinates of points

and parameters in Fig. 4(a) as shown in Table below. It is specified that distance between

diffuser plate and recording material as d.

In simulation, following conditions were applied for calculation simplicity.

1) An object light pulse scattered at O propagates in z-direction. This object light pulse and

reference light pulse simultaneously arrive at R0. Because optical path length of reference

light pulse is same as that of the object light pulse, interference fringes are generated at R0

and object light pulse is recorded at R0.

2) The cross sections are linear not only between illuminating light pulse and diffuser plate,

but also between reference light pulse and recording material.

3) The diffuser plate and recording material are placed parallel to each other.

Fig 4(a). Simulation model.



Definitions Of co-ordinates of points

In order to numerically obtain reconstructed image recorded at set of R whose y-

coordinate was 0 on recording material, we calculated set of coordinates of points at which

object light pulses, which were recorded at R, were scattered. We considered case in which

object light pulse scattered at O at TO when illuminating light pulse arrived at O. to-R0 was

expressed as

to-Ro = dc………………………………………………………..(1)

Then τR0 was given as,

τR0 = TO + tO − R0……………………………………………(2)

By considering (1), TRo expressed as

TR0 = τR0…………………………………………………(3)

Because the reference light pulse was incident to recording material at angle of θR , the cross

section propagated in x-direction. Then, vR , propagation speed of cross section between

reference light pulse and recording material, was given as

vR =c

sin θR…………………………………..(4)

Then TR0-R and TR were respetively expresssed as

tR0 − R =LR0−RvR

………………………………..(5)

τR = τR0 + tR0 – R……………………………….(6)



On the other hand, illuminating light pulse arrived at A0 at TA0 . Here, case in which the

object light pulse scattered at A. Because illuminating light pulse was incident to diffuser

plate at the angle of θO , vO , propagation speed of cross section between illuminating light

pulse and diffuser plate, was given as

vR =c

sin θ0………………………………..(7)

Then tO-A0 and TA0 were respectively expressed as

tO − A0 =LO−A 0vO

……………………………(8)

TA0 = TO + tO − A0……………………………….(9)

Object light pulse were scattered at A at TA0 and arrived at R at TR . Then, tA− R and TR

were, respectively, given as

tA− R=LA−Rc

…………………………(10)

TR = TA0 + tA− R……………………………….(11)

Here, if the object light pulse scattered at A is recorded at R, object light pulse and reference

light pulse simultaneously arrive at R. That is to say,

τR = TR………………………………..(12)

By applying (1)–(11) to (12), obtained following equation is:

d + LR0 − R sin θR = LO − A0 sin θO + LA− R……. (13)

If (13) is satisfied, object light pulse containing information on A is recorded at R. L R0

− R , LO − A0 and LA− R are given if the coordinates of A, R0 , and R are determined.And can

calculate a set of the coordinates of the points on diffuser plate that scatters object light

pulses recorded at R by examining (13) at a fixed R and a varying A and finding all

coordinates of A satisfying (13).Then image can be reconstructed from R. By changing

coordinate of R and conducting same calculations mentioned above, image reconstructed

from each point whose y-coordinate is 0 on recording material calculated.



According to the condition discussed: d =30 cm, θO = 0.5◦ , and θR , = 0.5◦ . The

sampling interval of A in both x- and y-directions was set to be 500 μm. Setup is to

difference between optical path length of object light pulse and that of the reference light

pulse to be shorter than coherence length of a femtosecond light pulse. Coherence length was

set to be same as the length of a 96 fs light pulse. Also, we assumed that size of the diffuser

plate was 40 mm (H) × 20 mm (V).

Fig. 4 shows simulation results of femtosecond light pulse propagation. From (a) to

(e), xR changed as follows: (a) – 2.0 mm, (b) –1.0 mm, (c) 0.0 mm, (d) 1.0 mm, and (e) 2.0

mm. X- and y-axes correspond to those of reconstructed images of femtosecond light pulse

propagation. Small dots composing each circle in Fig. 4 represent sampled points of a

femtosecond light pulse calculated, that is, set of points represent reconstructed image of

femtosecond light pulse propagating on diffuser plate. The reconstructed images move when

coordinates of R change along same direction in which reference light pulse swept and

curvature caused by principle of LIF holography.



Fig. 4(b) The time interval between adjacent pictures is 175 fs.

CHAPTER 5

OPTICAL SETUP

Mode-locked Ti:sapphire laser (Mira 900-D, Coherent, Inc.) to generate femtosecond

light pulses. Center wavelength and duration of the femtosecond light pulses were 800 nm

and 96 fs, respectively. Femtosecond light pulses were introduced into recording system of

LIF holography. Magnification of OB1 and OB2 were 20 and 10, respectively. The focal

lengths of CL1 and CL2 were 300 and 150 mm, respectively. We set the distance between

the diffuser plate and the recording material to be 30 cm.



Fig.5 Optical setup of experiment. (a) Schematic diagram of top view. M: mirror, HM: Half mirror, OB:

microscope objective, CL: collimator lens. (b) Photograph of the diffuser plate. A 1951 USAF resolution test

chart was patterned on the plate.

Illuminating light pulse and reference light pulse were incident on diffuser plate and

image sensor at 0.5◦ against normal of each plate, respectively. Plate is patterned with 1951

USAF resolution test chart as shown in Fig. 5(b) so as to easily recognize motion of

femtosecond light pulse propagation. We used a CCD camera (ORCA-HR, Hamamatsu

Photonics K. K.) to record a hologram. The number of pixels and pixel pitch of camera were

4000 (H) × 2624 (V) pixels and 5.9 × 5.9 μm, respectively.



Fig.5(c) Optical setup results

Seven digital holograms were extracted from

recorded whole hologram. The time interval between adjacent pictures is 96 fs.

Each image reconstructed from 512 × 512 pixels extracted from recorded whole

hologram consisting of 4000 × 2624 pixels. Brilliant circle is reconstructed images of

collimated femtosecond light pulse. By sequentially displaying Reconstructed images on a

monitor of a computer, and obtained moving picture of propagation of the collimated

femtosecond light pulse.

Square image in center of brilliant circle is zeroth-order diffraction image. The

reconstructed light pulse propagates from left to right when we move observation position

from left to right, which is same direction as that of the reference light pulse on the image

sensor on camera.

Actual time of phenomenon and time interval between adjacent pictures in Fig. 5(c)

were 576 and 96 fs, respectively. Thus, succeeded in DLIF holography using a femtosecond

pulsed laser.



CHAPTER 6

ADVANTAGES AND DISADVANTAGES

ADVANTAGES

By femtosecond pulse shaping, in which Fourier synthesis methods are used to generate

nearly arbitrarily shaped ultrafast optical wave forms according to user specification.



Applications of pulse shaping to optical communications, biomedical optical imaging,

high power laser amplifiers, quantum control, and laser-electron beam.

Reduce the spectral bandwidth of the diffracted signal. However, both theoretically and

experimentally that it is much easier in the frequency domain than in the space domain to

achieve a large enough diffraction bandwidth of volume holograms for the bandwidth of

100-fs pulses to be used for frequency-domain femtosecond pulse shaping.

The hologram is read out using forward-scattering phase conjugation to remove phase

distortion to all orders, including drift in the time of flight.

Dispersion of all orders is compensated by forming a dynamic spectral-domain hologram

of a signal pulse (that has a time-varying dispersion) referenced to a stable clock pulse.

DISADVANTAGES

It is a serial process and laser positioning is difficult.

There may be risk of cyto- and photo-toxicity.

CHAPTER 7

APPLICATIONS

Digital measurement of Three Dimentional Shapes.



The interferogram of the object under study is only registered for the object parts for

which the difference in path length between the reference and the object beam is less than the

coherence length.

Digital recording.

Digital audio and digital video is directly recorded to a storage device as a stream of

discrete numbers, representing the changes in air pressure (sound) for audio and chroma and

luminance values for video through time, thus making an abstract template for the original

sound or moving image.

Moving picture recording applications.


http://en.wikipedia.org/wiki/Time

http://en.wikipedia.org/wiki/Luminance

http://en.wikipedia.org/wiki/Color

http://en.wikipedia.org/wiki/Sound

http://en.wikipedia.org/wiki/Air_pressure

http://en.wikipedia.org/wiki/Discrete_number

http://en.wikipedia.org/wiki/Digital_video

http://en.wikipedia.org/wiki/Digital_audio


Moving pictures of collimated and converging light pulses and some images

extracted from them. Inherent feature appearing in the images essential to determine the

actual shape of the propagating light pulse.

CHAPTER 8

CONCLUSION



Recording and observation of infrared femtosecond light pulse propagation as a form of a

digital motion picture.

Center wavelength and duration of the light pulse generated from laser were 800 nm and

96 fs, respectively.

DLIF holography will contribute not only to the evaluation and improvement of

integrated optical devices, but also the observation, visualization and elucidation of

ultrafast phenomena in frontiers of natural science, optimum control of a laser beam in

laser processing, and so on.

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